The Respiratory System

We have already learnt how the circulatory system is responsible for supplying oxygenated blood to all parts of the body. We are now going to look at the respiratory system, which works hand in hand with the circulatory system.

The main function of the respiratory system is to allow oxygen to enter the body and for carbon dioxide to leave. This is called “gas exchange” and takes place on an internal level into tissues and an external level into the lungs. It is vital that it takes place for life to continue.

The circulatory system is constructed to allow this gas exchange to take place. Below are the organs within the system.

The Mouth allows an intake of air if there is a high demand or if the nasal passage is blocked in any way. It is an oval shaped cavity which is lined with a mucous membrane. The mouth contains the soft and hard palate, forming the roof of the mouth, as well as the gums in which the teeth sit. It is not ideal to continually breathe through the mouth as the air is not as well filtered and it can cause other medical problems.

The nasal cavity traps particles that enter the passages by containing shelf-like structures called turbinates. Any material that is deposited in the nose is transported by ciliary action to the back of the throat in around 10-15 minutes. The vascular mucus membranes of the nose will also warm and moisten the air as it is inhaled. The mucus which is produced will also be moved to the back of the pharynx for either swallowing or expectoration. The nose is formed by the two nasal bones and by cartilage and is divided by a septum. The nose also acts as a sounding chamber for the voice as some of the bones surrounding the nasal cavity are hollow. These hollows are called paranasal sinuses and allow the voice to become resonant, lighter and to secrete mucus to help with air filtration. The olfactory receptors are found in the nasal cavity, where the nerves connect directly with the inha and have a powerful and immediate effect on emotions.

The pharynx (throat) is a muscular cavity that begins from behind the nose to the beginning of the voice box and the oesophagus. The pharynx is divided into three sections. The nasopharynx lies behind the nose and can be seen when the mouth is wide open, the oropharynx which lies behind the mouth, and the laryngopharynx which lies behind the larynx. The upper part of the pharynx lets air pass through, whilst the lower parts permit air, foods and fluids to pass. When it is necessary to swallow, breathing will stop as the oropharynx becomes blocked off from the nasopharynx as the soft palate is raised, as it is impossible to be able to breathe whilst swallowing.

The larynx, also known as the voice box, is a 2” tube shaped structure which is located at the entrance of the trachea. The larynx contains two vocal cords, which will vibrate together when air passes between them. This gives us the sound of the voice. The larynx is made up of several irregular cartilages and the lobes of the thyroid gland are on either side. The oesophagus, which is the tube that carries food from the mouth to the stomach, is just behind the trachea and the larynx. Both openings of the oesophagus and the larynx are close together in the throat, so when the act of swallowing occurs, a flap called the epiglottis keeps the food out of the windpipe by moving down over the larynx.

The trachea, also known as the windpipe, is a tube-like structure consisting of between 16 – 20 rings of cartilage that joins the nose and mouth to the lungs. It measures approximately 10-12” in length and runs from the lower part of the larynx to the lungs by dividing into the bronchi. The trachea contains an epithelial lining that secretes mucus, which traps any dust. It is then swept upwards by the cilia towards the larynx away from the lungs.

The bronchi are supported by cartilage and are formed when the trachea forks into two branches, making up the left and right bronchi. These branches then divide again, with the right Bronchus being wider and shorter than the left. The right bronchi then divide into two branches for the middle and lower lobes. The left bronchi is nearly double in length, being 5cm long and divides again, one for each Broncho-pulmonary segment. Within the lungs, the bronchi divide again into smaller bronchi, called bronchioles. There are numerous glands in the wall of the bronchi which secrete slimy mucus, which helps to trap dust and any other particles, which are then propelled upwards to the mouth by cilia.

The bronchioles are the first divisions of the bronchi that no longer contain cartilage but are made up of a single layer of epithelial cells. The bronchioles are smaller than one millimetre in diameter and control the air distribution into the lungs. The bronchiole end in the alveoli.

The alveolar sac contains around 300 million alveoli, which are arranged in grape like clusters to increase the surface area, which can become reduced due to irritants such as dust. It is here that gas exchange takes place. To allow this to happen, the alveoli are constantly moist and are surrounded by a network of capillaries. Oxygen is in a higher concentration in the alveoli than in the blood and so therefore it is able to diffuse into the blood through a thin layer of cells. The reverse happens with carbon dioxide, which is a higher concentration in the blood than the alveoli and so it diffuses into the alveoli through the thin layer of cells.

The lungs are located in the thorax and are cone shaped. They make up one of the largest organs of the body with a huge surface area. The main role of the lungs is to exchange gas; oxygen for carbon dioxide and on average a person breathes 25,000 times a day, moving 10,000 litres of air a day.

Mucus is produced in the lungs that traps any inhaled particles, which can be removed by coughing.

The lungs are situated in a space, known as the pleural cavity. Each lung is covered in two thin layers of a single celled membrane called pleura which slide back and forth over each other every time a breath is taken to allow the lungs to expand and contract. There is a small amount of fluid here to prevent friction. The pleura, which are connected to the chest wall, are called the parietal pleura, and the pleura that are attached to the lung are called visceral pleura.

The front and back of the lungs are protected by the ribs, and the intercostals muscles help allow the chest wall to move. The front of the ribs contains costal cartilage which connects the sternum and the ends of the ribs. The back of the lungs contains the transverse processes of the thoracic vertebrae. The lungs differ on either side with the right lung having 3 lobes; the superior, middle and inferior lobe and the left lung only having the superior and inferior lobe.

The Diaphragm is a dome shaped muscular sheet that extends along the bottom of the rib cage and inserts into the lower ribs. The diaphragm relaxes during inhalation to allow more room in the thoracic cavity, which in turn creates a suction to allow air to be drawn into the lungs. When you exhale, the diaphragm expands which reduces the amount of space in the cavity for the lungs, which forces the air out.

The Intercostal Muscles occupy the space in-between the ribs and are made up of two types. The internal muscles are on the inside of the rib cage and extend from the front of the ribs and go around the back, and the external muscles are on the outside of the ribs and cover the back of the rib, around to the bony part at the front. They receive messages from the brain to control breathing and are responsible for working alongside the diaphragm.

Breathing Mechanism

To be able to take in oxygen and allow carbon dioxide to be expelled, a complex procedure needs to take place.


  • The diaphragm contracts and moves downwards
  • This forces the rib cage muscles to contract
  • The ribs then move up and out
  • There is decreased pressure in the chest
  • The air is sucked down into the lungs through:
  • Nose, pharynx, larynx, trachea, bronchus, bronchiole and to the alveoli.
  • Once the oxygen is in the alveoli, gas exchange takes place so that the carbon dioxide is ready to be exhaled. The reverse then happens.


  • The muscles of the diagram and intercostals relax
  • The size of the thorax reduces
  • Air is forced out of the lungs

Gas Exchange

  • Once the air that we have inhaled reaches the lungs, the 21% of dissolved oxygen then diffuses through the alveolar lining cells of the alveolar and the walls of the capillaries and enters the plasma of the blood.
  • From the plasma, the oxygen then diffuses into the red blood cells (erythrocytes) and combines with the hemoglobin to form oxyhemoglobin.
  • The newly oxygenated blood then leaves the capillary network and enters the pulmonary veins to be transported back to the heart to be pumped around the body for its use.
  • Once the oxygen has travelled the body, the deoxygenated blood leaves via the capillary network from the pulmonary artery back into the alveoli.
  • The exhaled breath still contains 16% oxygen and 4 ½% carbon dioxides.

Breathing Patterns

Shallow Breathing

When we take short intakes of breath, the Intercostal muscles around the ribs tend to work harder than the diaphragm, which in turn can cause the diaphragm to become weak. Stress and tension can be the cause of shallow breathing and it can lead to a lack of oxygen entering the body, as well as constricting the chest and lung tissue.

Deep Breathing

By using the diaphragm muscle, we are able to fully fill our lungs with air and therefore take in the largest amount possible. The abdominal muscles also play an important role in deep breathing.

Pathologies of the Circulatory System

DiseaseSigns & SymptomsCause
EmphysemaShortness of breath due to obstruction.Permanent damage of the lungs due to smoking or working in an environment with chemicals.
BronchitisBurning sensation during breathing, cough, sore throat.An infection of the airways caused by virus or bacteria.
PneumoniaCold feeling, difficulty breathing, cough, fever.Inflammation of the tissue within the lungs.
TuberculosisPersistent cough, weight loss, night fevers.Bacterial infection, usually affecting the lungs but can affect other body systems.
RhinitisItching and sneezing and irritation of the nose.Inflammation inside the nose, usually due to an allergy.
LaryngitisSore throat, pain in the voice box, mild fever.Inflammation of the larynx (voice box) due to infection or damage.
PharyngitisSore throat.Bacterial or viral infection.